The possibility of determining the position of a mobile device in a wireless telecommunication network has enabled application developers and wireless network operators to provide location based, and location aware, services. Examples of those include guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services giving the mobile user information about their surroundings.
In addition to these commercial services, the governments of several countries have put requirements on network operators to be able to determine the position of an emergency call. For instance, the governmental requirements in the USA (FCC E911) state that it must be possible to determine the position of a certain percentage of all emergency calls. The requirements make no distinction between indoor and outdoor environments.
In outdoor environments, the position estimation can be done using positioning systems, e.g. GPS (Global Positioning System) based methods like Assisted-GPS (A-GPS). Position estimation can also be performed using the wireless network itself. Methods using the wireless network can be arranged into two main groups: those using measurements from a single radio base station, and those using measurements from a plurality of radio base stations.
The first group comprises methods that are based on the radio cell to which a mobile terminal is attached, e.g. by using Cell-ID or a combination of cell-ID and Timing Advance (TA). The TA measurement principle is depicted in FIG. 1.
A radio base station 10 serves three radio cells 12a, 12b, 12c. Although three cells are depicted in this example, in general each radio base station will serve one or more radio cells. In order to determine the location of a mobile terminal 14, the travel time of radio waves from the radio base station 10 to the mobile terminal 14 and back is measured. The distance r from radio base station 10 to mobile terminal 14 then follows from the formula:
  r  =      c    ⁢          TA      2      where TA is the round trip time and where c is the speed of light.
The round trip time measurement alone defines a circle, or if the inaccuracy is accounted for, a circular strip around the radio base station 10 (more accurately, a sphere, or spherical shell) is defined. By combining this information with the cell polygon, angular extent of a part-circular strip 16 that defines the possible position of the mobile terminal 14 can be computed.
In several systems, therefore, among those Release 8 of the 3GPP specifications (also known as long term evolution, or LTE), the round trip time TA can be used to identify the distance from the antenna at which a mobile terminal is positioned. However, it is not possible using this method to ascertain where exactly in the sphere or sector the UE is. If TA measurements determine that the mobile terminal is for example 500 m from the radio base station, this is along an arc in a sector or circumference of a circle.
To overcome this problem, a second group of methods uses round trip time measurements from a plurality of radio base stations. By determining its distance from a plurality of radio base stations, a mobile terminal can more accurately triangulate its position.
However, modern telecommunications systems are designed to provide high data rates in the downlink and the uplink (i.e. in communications to and from the mobile terminal). It is also desirable to reduce power usage in the mobile terminal, in order to prolong the battery life as much as possible. Both of these requirements mandate a high quality radio link between the mobile terminal and its serving radio base station (i.e. the radio base station associated with the mobile terminal's serving radio cell). Thus, interference from other neighbouring radio base stations should be kept to a minimum, and in modern telecommunication systems this is very successfully achieved. A mobile terminal wishing to determine its location, however, has difficulty in detecting signals from neighbouring radio base stations for this very reason.
What is required, therefore, is a method whereby a mobile terminal can detect signals from radio base stations other than its serving radio base station, for example, in order to determine its location.